In gravure printing or rotary screen printing that use a cylindrical printing plate (printing cylinder), it is possible to print an endless pattern on a printing medium by repeatedly printing a unit pattern that covers the entire periphery of the cylindrical surface.
An example of recording a unit pattern on a gravure printing cylinder is illustrated in FIG. 2, which is prepared to explain the preferred embodiments of the present invention later but has the same structure in an input section 11 and in an output section 12 as the conventional apparatus.
An original picture is fixed on an input cylinder 111 in the input section 11, and the input cylinder 111 and a printing cylinder 121 in the output section 12 are rotated synchronously. The input cylinder 111 is driven by a motor 112, and the printing cylinder 121 is driven by another motor 122. The rotative positions of the input cylinder 111 and the printing cylinder 121 are respectively detected by rotary encoders 113 and 123. With the signals from the rotary encoders 113 and 123, a controller 15 controls the motors 112 and 122 to synchronize the rotations of the input cylinder 111 and the printing cylinder 121. A pickup head 114 is provided to move along the axis of the input cylinder 111 on a ball-screw 115 which is driven by a motor 116. A recording head 124 is similarly provided to move along the axis of the printing cylinder 121 on another ball-screw 125 which is driven by another motor 126. Another set of rotary encoders 117 and 127 are provided for respectively detecting the positions of the pickup head 114 and the recording head 124. The controller 15 synchronizes the movement of the two heads 114 and 124 with the rotations of the cylinders 111 and 121. While two-dimensional scanning is effected using such mechanism in the input section 11 and in the output section 12 respectively, the image of the original picture is read with the pickup head 114 in the input section 11 and the image is recorded on the printing cylinder 121 with the recording head 124 in the output section 12.
The input cylinder 111 and the printing cylinder 121 are rotated synchronously in the above explanation. It is not necessary, however, to rotate them simultaneously: the image data thus input can be stored in a memory (RAM in the controller 15, for example) once, and then the image data is restored from the memory at another time, where the rotation of the printing cylinder 121 is synchronized with the rotation of the input cylinder 111 when the image was read.
The recording head 124 normally moves intermittently along the axis of the printing cylinder 121. That is, for a rotation of the printing cylinder 121, the recording head stays stationary and records a line of the image (i.e., engraves a linear array of cells) as shown in FIG. 4A. The direction of the rotation of the printing cylinder 121 is hereinafter referred to as the primary scanning direction and the line is referred to as a primary scanning line. Then the recording head 124 moves by a unit length to the next position along the axis of the printing cylinder 121, and stays there for another rotation of the printing cylinder 121 to record the next line of the image. The direction of the movement of the recording head 124 is referred to as the secondary scanning direction.
If a unit pattern is to be recorded, record-starting points of adjacent primary scanning lines should be at the same rotative position. The intermittent movement of the recording head 124 described above requires the time for the recording head 124 to move by the unit length from one primary scanning line to the next primary scanning line, and causes an idle rotation of the printing cylinder 121 between the adjacent recording lines. When, therefore, the ring recording method as shown in FIG. 4A is adopted to record a unit pattern constituted of M primary scanning lines, the printing cylinder 121 must rotate 2.times.M times.
An improved method is proposed in the U.S. Pat. No. 4,013,829 in which the record-starting points of adjacent primary scanning lines are displaced in the primary scanning direction (or in the rotative direction) regarding the time necessary to move the recording head, whereby the idle rotation between lines is eliminated.
Because, for speedier recording (engraving), the engraving stylus of a gravure recording head is always vibrated by resonance, the depth of the cells is shallower than normal at first when the engraving stylus is once lifted and then dropped to record a new line. The Japanese Examined Patent Application No. 56-16074 shows a double engraving method in which a primary scanning line is recorded 1+.alpha. rotations to engrave the first cells twice to obtain the normal depth.
In the two methods, the printing cylinder 121 idles while the engraving stylus is lifted, moved to the next recording line and dropped to record the next line (in the latter double engraving method, the double engraving time is added).
A more efficient method is the helical recording method. In the helical recording method, the recording head is moved continuously in the secondary scanning direction while the printing cylinder 121 is rotated, whereby the cells are engraved helically on the printing cylinder 121 as shown in FIG. 4B. In this case, the idle rotation of the printing cylinder 121 is eliminated.
There is still a problem in recording a unit pattern on the printing cylinder 121, whether by the ring recording method (FIG. 4A) or by the helical recording method (FIG. 4B). There are two kinds of endless patterns: a) an endless pattern of a narrower sense in which an independent and separate element or elements lie on a uniform background forming a unit pattern, and the unit pattern is repeated endlessly; and b) a seamless pattern in which no independent element exists or no uniform background exists between elements of the pattern. In case of the endless pattern of the narrower sense a), a natural endless pattern (which means that the joining line between adjacent unit patterns is invisible) can be formed by cutting out the unit pattern with such two parallel (but not necessarily straight) lines that pass through the uniform background and by joining the unit pattern repeatedly. In case of the seamless pattern b), a discrepancy is inevitable at the joining line of the unit patterns and the joining lines look apparent because no such lines exists that passes exclusively through the uniform background. Thus producing a natural seamless pattern is difficult.
Japanese Unexamined Patent Application No. 57-29457 (Examined No. 4-24225) discloses a method in which a unit pattern a little longer than the periphery of the printing cylinder is prepared, and the image data in the excessive length (the overlapping portion) of the unit pattern is gradually changed to smoothly join the adjacent unit patterns. Further, the International Publication WO 86/00770 discloses a method using an indented mask corresponding to the pattern in order to camouflage the joint. Though, in these methods, steep change at the joint can be avoided, the image at the joint looks unnatural because the image at the joint is constituted of an overlapped double image of the adjacent unit patterns and the image is blurred. Further, in the method of WO 86/00770, the shape of the mask must correspond to every specific pattern so that preparation of the mask is troublesome.
The above problem is not limited to recording a unit pattern on the gravure printing cylinder, but a similar problem occurs in recording a unit pattern on the printing cylinder of a rotary screen printing. Japanese Published Examined Patent Application No. 57-11781, Unexamined Patent Application No. 2-63741 etc. describe about producing a rotary screen printing.